isArray not narrowing a type variable correctly since 5.8

[jakebailey]

🔎 Search Terms

isarray 5.8

🕗 Version & Regression Information

• This changed between versions 5.7 and 5.8

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.2#code/KYDwDg9gTgLgBAYwgOwM72MgrgWzgYQEMZgBzaATzgG8BYAKDibh2BkIBNjC4BeOAEQB9Ia3Zd2AgDQNmcGBDAAbYADdgSvoIXK1G6bOYwAlqyXHkwLQI5qFEJagB0JsxeAHGzBGCwAFKAgAM2MVKGtVAA4nHywhMECQsM85DmNUQgAjFQ4AIQoAEWAgwiwlGCFXDXdrNIzs4A4AWkyKJtsSsph2uwgHZyrzSxTmOqyc-KLO8qFYgODQ4HD+G3Txxpa2jtLypqiY33jExahPAF8GBlBIWHkKMCsiEnIoY2BUAB58OFASZA5UARiGRKFoniCoBQAHx8QxMAA+cAAFEj0K9kKQ4IjVBBjBwAJRwABkNDgAG0BHBZhAcDhCABlYBgQhQYEccEvCgCAC6AC4CABuOBnfFwrGCy70K7gaDwIJYZAIEwoOAAK1xyA50Den2+v0wAKBz0oUKRCGBLx1-PwZO5+OtFu17y+MLoXiYUDYWCgyEQjte7yc6osSOkAkJhEBWoDuqhAoYFyl9Gusrg8sVyt9CmjOrN-qtRohOvFns4KCUVGjFFt9sLlveNDFnpg3t9AEEoKyKE50h2u3njTHRe65HAAPxqjU594DovvYej5j882DnXx+iJoA

💻 Code

export const enum Category {
    metadata = "__metadata",
    toplevel = "toplevel",
    timeline = "devtools.timeline",
    cpuProfiler = "v8.cpu_profiler",
    disabledByDefault_timeline = "disabled-by-default-devtools.timeline",
    disabledByDefault_cpuProfiler = "disabled-by-default-v8.cpu_profiler",
}

export type Categories<C extends Category = Category> =
    | ((string | void) & { [" _commaSeparatedCategory"]: C; })
    | C;


export function joinCategories<C extends Category>(categories: C[]): Categories<C> {
    return categories.join(",") as Categories<C>;
}

export function toCategories(categories: Categories | readonly Category[]): Categories {
    return Array.isArray(categories)
        ? joinCategories(categories)
        : categories;
}

🙁 Actual behavior

5.8 and 5.9 say narrow categories to any[] instead of readonly Category[], and the else side is still the union, leading to an error.

🙂 Expected behavior

Categories should be the right array type, and the else branch should just be Categories.

Additional information about the issue

Noticed this while updating typescript-benchmarking past 5.7.

I think I remember an Anders PR in 5.8 that is related to this, but I can't remember what it is anymore.

[Andarist]

Bisects to #56941

The isArray narrowing has not changed at all (you can verify that by hovering over categories in both branches on both versions of TS). The problem is that the first branch of this ternary infers this signature function joinCategories<any>(categories: any[]): any. So in TS 5.7 the whole ternary just collapsed to any - but since this PR all branches are checked separately.

[jakebailey]

Oh wow, huh, yeah.